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Tightfisted VCT

Variable Camshaft Timing (like BMW VANOS) from a tightfisted person.
  [vote for,

In case you didn't already know; tightfisted = extremely unwilling to spend money.

Basically, this varies the camshaft timing by playing with the slack in the cam chain.

I've noticed that most modern car engines employ a chain to drive the cams, this chain has a tensioner which is often just a curved bit of plastic that is pressed against the chain to take up any slack. It occured to me that the slack in the chain could be taken up in one of 3 positions as it whirls round the 3 sprockets (clockwise, for the purpose of mental imagery)- between the crankshaft and intake camshaft (C-I), between the camshafts (I-E) and between the exhaust camshaft and crankshaft (E-C). It then occured to me that the relative angles of each of the sprockets could be changed by moving the slack around between these 3 positions. Watch as the idea becomes incomprehensible the very moment I try to explain it :P

Imagine a camchain-end view of an engine (intake on the left, exhaust on the right), the crankshaft is fixed and the camshafts are free to rotate (some people with sufficiently old Fiats may have experienced this) and there is an equal amount of slack between each sprocket. Now, if you pull on the C-I section of chain to take up all the slack the camshafts will have to rotate towards the left, retarding the cam timing (for an engine that rotates clockwise from this viewpoint). Conversely, if you pull on the E-C section the camshafts will rotate to the right, advancing the camshaft timing. In the last case, taking up all the slack in the I-E section will make the camshafts rotate towards each other, advancing the intake cam whilst retarding the exhaust one. By taking up different amounts of slack in each section (using the control inputs to the tensioners) one can achieve various combinations of intake and exhaust camshaft adjustment e.g. taking up equal slack on the C-I and E-C sections will rotate the camshafts away from each other while maintaining the same amount of slack on the C-I section while taking up all the remaining slack in the I-E section will rotate the exhaust camshaft towards the intake one while the intake camshaft remains stationary. *watches as other halfbaker's expressions become triple-glazed*

It's a fair bet that this won't work because it's so stupidly simple that someone would've come up with it already and we'd all be driving around in cars with VCT.

squigbobble, Apr 29 2005

Toyota MR2 http://www.mr2sc.co...es/articles/vvt.htm
variable cam timing mechanism via movable tensioner [Freefall, Apr 29 2005]


       Yes, it is stupidly simple. So stupidly simple that Porche used this method in some of their engines.   

       Still looking for the Porche link, but I found a nearly identical implementation for the Toyota MR2. (see link)
Freefall, Apr 29 2005

       Thanks for the link [FreeFall], the Porsche version works in exactly the way I envisaged but it's kind of a baby version, my version would allow independant control of both camshafts.   

       [Pa've] The engine design you're thinking of is one using solenoids to control the valves, this is being researched at the moment but car electrics have to change up to 42V because 12V (or 14V) doesn't provide enough power. I think the other idea you were thinking of is well baked in the guise of diesel/electric trains, these use a diesel engine running at a constant speed to run a generator and then propel the train with electric motors.
squigbobble, Apr 30 2005


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